The essential region of the LTR responsible for enhancement of the transforming activity of v-mos and activation of the transforming potential of c-mos is a sequence in the unique 3' region of the LTR containing the 73-bp tandem repeat. A single copy of the tandem repeat does enhance the transforming activity of v-mos, while transfection of DNA containing less than one copy of the 73-bp sequence result in a transforming activity equivalent to transfection of v-mos alone. A sequence within the normal mouse DNA preceding c-mos prevents activation of the transforming potential of c-mos by a 3' LTR. A series of deletion mutants were utilized in defining a 200 bp DNA fragment, 1.3 kb upstream from c-mos, that functions in a cis-acting fashion to prevent activation of c-mos. Insertion of this cis-acting element into the viral sequences preceding v-mos prevents enhancement of the transforming activity of v-mos by a 3' LTR.